Friction materials are used in various applications, such as brake pads, brake linings, clutch facings, and brake shoes of automobiles, industrial vehicles, railcars, airplanes, etc. Resin compositions generally used for making these friction materials are produced by employing a phenolic resin as a binder and blending together the phenolic resin, fibers such as glass fiber, aramid fiber, or metal fiber, and an inorganic filler such as calcium carbonate or barium sulfate.
Patent Literature 1 discloses a phenolic resin composition preferably usable for friction materials etc. The cured molded product made from this composition has good heat resistance despite the fact that it is made of a phenolic resin composition, but its flexural strength after applying thermal history at 350° C. for 8 hours, for example, drops to 43% with respect to its normal-state flexural strength (see Example 1). In recent years, further improved properties are being demanded of friction materials so that they can be used in severer conditions.
Patent Literatures 2 and 3 disclose the disadvantages of using, in friction materials, flame retardants such as antimony compounds and reinforcement materials such as asbestos fibers, which have been employed for improving heat resistance and flame resistance of friction materials.
Accordingly, there has been a demand for friction materials and resin compositions for friction materials in which the binder itself has further improved heat resistance and mechanical properties.
Patent Literature 4 discloses the possibility of using, as linings etc. of brakes and clutches, a filled article made using a mixture of particles of a high-molecular-weight linear aromatic polyimide containing a ketonic carbonyl group and particles of a low-molecular-weight linear aromatic polyimide having an amine end group. The article, however, is not preferable in terms that molding is complex and requires severe conditions, such as a compression process under a pressure of 35 MPa at around 400 to 420° C. or under a pressure of 690 MPa at room temperature, and heat treatment at 400° C. for about 3 hours followed by heating at around 425 to 435° C. for 1 hour.
Patent Literature 5 discloses an aromatic imide oligomer having an addition-reactive group at an end thereof. The document discloses a method of producing a fiber composite material using a solution of the aromatic imide oligomer. However, the document is completely silent about using the oligomer for a friction material.
Patent Literature 1: JP 2003-292728 A
Patent Literature 2: JP 2002-173667 A
Patent Literature 3: JP 2001-240847 A
Patent Literature 4: JP 61-36010 B
Patent Literature 5: JP 2000-219741 A